The real problem with traditional flowbench testing is simple: it does not directly measure the thing the engine cares about. Bench CFM is a comparison based on pressure drop through a known orifice, not a direct reading of the actual air moving through the part. That was workable when density changes were smaller, but it falls apart once temperature, pressure, humidity, and especially boost start changing the weight of the air inside the same volume. What matters to the engine is air mass. Air is compressible, so the same cubic feet per minute can represent very different amounts of oxygen depending on density. That is why modern engine control strategies use mass airflow, not just volume. By adding a mass airflow sensor and monitoring the conditions around it, we turned the flowbench from a ratiometric CFM comparator into a direct air-mass test tool. With Banks iDash Pro handling the real-time sensor data and logging, we can see airflow in terms that actually correlate to what the engine uses.
The video opens with a simple comparison: one exhaust may flow more air, create less backpressure, and therefore make more horsepower than another. The real question is how to prove that one part actually flows better. In the automotive industry, the usual tool for this job is a flow bench, which is commonly used to evaluate airflow in air intakes, cylinder heads, manifolds, and exhaust components. Results are typically expressed in cubic feet per minute, or CFM.
Eric argues that this common practice misses a more important issue. A flow bench can be useful, but the industry often focuses on the wrong output. The episode sets out to explain what a flow bench really measures, what it does not measure, and how Banks modified its own bench so it could provide data that better reflects how air behaves in modern engines.
To understand the limitations of a flow bench, the recap first establishes some basic airflow concepts. Air pressure is force exerted over an area, usually expressed in PSI. Ambient air pressure at sea level is about 14.7 PSI, but people do not feel it because that pressure acts equally in every direction. Air only moves when there is a pressure differential. When pressure is higher on one side and lower on the other, air flows from high pressure to low pressure in an attempt to equalize.
That principle is what makes an air gun work: compressed air in the tank is at higher pressure than the surrounding atmosphere, so opening a path between them causes flow. A flow bench works from the same basic idea. It measures the pressure differential across a component, and because the pressure decreases as air moves through a restriction, that differential is described as a pressure drop. Measuring pressure drop through an intake, manifold, or exhaust is one step toward understanding how that part will behave on a vehicle.
CFM describes volume over time: the number of cubic feet of air passing through a boundary each minute. Engines are often discussed in CFM because engine displacement is a fixed volume, and RPM determines how quickly that volume is pumped. That makes CFM a convenient shorthand, but Eric emphasizes that it is not a complete description of airflow.
Modern engines do not fuel themselves based on volume alone. They calculate air-fuel ratio from air mass. Mass airflow is the weight of the air moving through the intake, engine, and exhaust over time. CFM is volume over time; mass airflow is weight over time. Because air is compressible, the same mass can occupy different volumes depending on pressure, temperature, and humidity. One pound of air at 10 PSI can be compressed to 100 PSI and occupy one-tenth the volume, assuming temperature and humidity remain constant. The volume changes, but the weight does not.
That distinction matters because an engine cylinder has a fixed volume. What determines how much oxygen is actually in that volume is air density. Eric illustrates this with two coffee cans: one filled with popped popcorn and the other with unpopped kernels. Both contain popcorn, but the kernels are much denser. Air behaves similarly. When air heats up, it expands and becomes less dense. When it cools, it becomes denser, meaning the same volume contains more oxygen. More oxygen in the same cylinder volume allows more fuel to be burned and more power to be produced.
The video then walks through the physical layout of the Banks flow bench. At the top is a precision-ground adapter plate that allows different adapters to be mounted for whatever component is being tested. Beneath that is a chamber containing flow-straightening mesh. The purpose of the mesh is to promote laminar flow across the cross section so the readings remain consistent.
The bench uses 14 vacuum motors as its pumping source. Above and below the pumps are cutouts with rotating plates. By changing which openings are exposed, the airflow direction can be reversed so the bench can simulate either intake or exhaust flow depending on the part under test.
Historically, pressure drop on a flow bench was measured with a water-column manometer. A manometer is a simple but very sensitive device that shows small pressure changes by the amount of water displaced by pressure or vacuum. Eric demonstrates that even a very small vacuum, around one-tenth of a PSI, moves the water noticeably. One inch of water equals 0.036 PSI, which explains why flow bench results are traditionally reported in inches of water rather than PSI. The water scale is sensitive enough to resolve the small pressure differences that matter in airflow testing.
A key point in the episode is that a traditional flow bench does not directly measure CFM. It measures pressure drop. To convert that pressure information into an airflow number, the bench needs a reference. That reference comes from calibration plates, also called known orifices. These plates are certified to flow a known CFM at a known pressure drop.
Banks uses SuperFlow calibration plates for this purpose. The known airflow characteristics of those plates are used to calibrate an internal variable orifice inside the bench. Once that internal orifice has been characterized, the bench can compare the pressure drop across the test component with the pressure drop across the calibrated internal orifice. From that relationship, it calculates airflow.
This is a ratiometric measurement. In other words, the bench is not directly measuring the actual volume of air flowing through the component. It is inferring airflow by comparing one pressure-drop relationship to another. Eric summarizes this by noting that SuperFlow itself describes bench CFM as not being the actual volume of air flowing through the bench. The bench compares the characteristics of the unit under test to a reference orifice inside the machine. That is why the result is called bench CFM rather than a direct airflow measurement.
Eric argues that the larger problem is not just that bench CFM is indirect, but that CFM itself is the wrong metric for many modern applications. CFM ignores what is inside that volume. It does not tell you how much the air weighs or how dense it is. Since density changes with temperature, pressure, and humidity, the same CFM can represent very different amounts of oxygen.
That limitation was less severe when flow benches first became common in automotive development during the 1960s. At that time, many engines had relatively little restriction between the intake and the cylinder, and turbochargers and superchargers were uncommon. Under those conditions, air density varied less dramatically, so CFM was a more acceptable approximation.
Modern engines are different. Eric states that about 50 percent of new vehicles now use some form of forced induction. Turbochargers and superchargers compress incoming air to increase air mass in the cylinder. Forced induction therefore changes air density substantially. Once density changes become significant, a volume-based metric becomes much less meaningful. In his view, the compressibility of air is the biggest failing point of the traditional flow bench, and it makes CFM-based testing obsolete for many current applications.
Banks' solution was to modify the flow bench so it measures mass airflow directly. The core change was adding a mass airflow sensor in line with the test setup. In most diesel trucks, the MAF sensor is located in the intake tube, and it uses electrical current to determine airflow. Eric simplifies its operation by describing a small heated wire placed in the airflow path. As air passes over the wire, it changes the wire's temperature. The electronics use that cooling effect to calculate the mass, or weight, of the air moving through the sensor. Denser air cools the wire more strongly.
The Bosch sensor used in the setup also measures temperature, pressure, and humidity, which are the three variables needed to characterize air density. Because mass is conserved throughout the system, the mass airflow measured upstream corresponds to what is entering the combustion chamber. That also makes the reading directly relevant to how an engine control module calculates air-fuel ratio. In other words, MAF is not just a more direct measurement than bench CFM; it is also more representative of the parameter the engine itself actually uses.
With this arrangement, the bench no longer depends on calibration plates, ratiometric calculations, or CFM conversions. It becomes, in Eric's words, essentially a glorified air pump with direct mass-flow measurement added to it. The result is real-time airflow data based on mass rather than inferred volume.
Banks did not stop at simply inserting a MAF sensor into the airflow path. The team also added temperature and pressure sensors both before and after the MAF sensor. This allows them to observe how the sensor itself affects flow in an intake system. When the MAF is installed in a component that normally uses one, its presence reflects real vehicle operation. However, when testing parts such as an exhaust or an intercooler, the sensor is not normally part of the system, so its influence has to be accounted for.
By measuring the density differential across the sensor, Banks can factor out the sensor's own effect in the final calculations. That improves accuracy when testing components that would not ordinarily contain a MAF sensor. To further stabilize the data, the setup also monitors ambient conditions with an Airmoist portable weather station, recording ambient temperature, pressure, and humidity.
The full system is displayed through four iDash units, which show the densities in the various components in real time. Eric notes that four iDash displays are being used in the demonstration, but a single iDash could also data-log all of the sensors. The goal is to capture the complete density picture inside the bench and in the surrounding environment so tests remain comparable from one session to the next.
The conclusion is blunt: a CFM-based flow bench has been outdated for decades. According to Eric, bringing it into the present does not require reinventing the entire machine. It requires adding direct mass-airflow measurement and the supporting sensors needed to account for density and environmental conditions.
With the modified setup, Banks can directly compare how air moves through its own components and competing parts under controlled and repeatable conditions. The emphasis shifts away from bench CFM and toward mass airflow, density, and pressure behavior, which are more relevant to modern engines and especially to forced-induction applications. The episode closes by framing this modified bench as a more useful engineering tool for evaluating real airflow performance rather than relying on a traditional number that may no longer describe what matters most.